Deep sand bed strategies have opened up entirely new dimensions for keeping previously challenging saltwater fishes, plants and invertebrates.

The use of deep substrates in the marine aquarium has evolved remarkably in the last decade. In the broadest definitions of the strategy, an aquarist can use a wide range of media and grades. The most popular application to date has been with fine aragonite sand at static depths of 3” (7.5 cm) or more. Some folks have also incorporated “plenums” under their substrates with hope for added benefits (more about this below). Without a plenum or any other modification, however, the methodology is still simple, natural and can be quite beneficial to the aquarium.

Deep sand beds, also known as DSB’s, have many benefits beyond the aesthetic. Indeed, there are an increasing number of sand products available to help replicate most any biotope imaginable: snow white and sugar-fine oolitic material from the Caribbean, black sands alike Hawaiian or Indonesian beaches, pink sands that are reminiscent of a Samoan paradise, and many other natural media that include attractive whole and broken shells, for example.

Compositionally, any substrate you choose is almost certain to be made of aragonite or calcite. Both are forms of calcium carbonate, but arguably aragonite is to be favored for it’s better solubility and buffering capacity in seawater (providing necessary bio-minerals, buffering pH, and supporting calcification). Aragonite can begin to dissolve, in fact, at a high pH over 8.0 (a still safe level for marine life), while calcite does not readily dissolve until the pH falls well below 8.0. This means that calcite is not likely to impart any significant benefits (buffers/alkalinity) into the water until the pH falls to a level that is too dangerous for most marine life. In this regard, the old argument of dolomite & crushed coral versus non-calcareous freshwater "gravel" for marine aquariums in the early days was a moot point (they were all calcite).

The extent of calcareous sand benefits, in part, is influenced by the grain size of the media. To generalize, finer grains dissolve more readily and afford greater biotic activity collectively. Course sands and gravel, on the contrary, are slower to dissolve while being more likely to trap detritus to excess. Any grain size desired though can ultimately be managed and kept “healthy” with adequate water movement in the system, and sand-stirring (naturally, by life forms in the tank, or assisted by the aquarist).

One of the principal benefits to deep sand bed methodologies is the potential for natural nitrate reduction (NNR). The speed and extent to which a DSB can reduce nitrate in an aquarium amazes many aquarists… especially those that have struggled with nitrate control for any length of time through other means. The benefits to an established tank with the upgrade to a DSB are apparent in just a few weeks with potential for complete nitrate reduction (to “zero”)!

Many theories and recommendations abound as to how best NNR can be achieved. We do know that good denitrification is less likely (but not impossible) to occur in shallow sand beds (less than 3”/7.5 cm), and with course sand. Some of the difficulties and criticisms of failed DSBs may be fairly attributed to such intermediate sand depths (1-3”/25-75mm) where the substrate was neither deep enough for efficient denitrification, nor shallow enough for thorough nitrifying activities. This condition is mitigated by the all too common lack of adequate water flow in some marine aquaria. Strong water flow is critical in most any marine aquarium. Aspire to provide at least 10 to 20 times the tank’s total volume per hour in water flow for your aquarium. The ocean is a dynamic environment! When nitrate control is your primary ambition, use deep fine beds of sand. Smaller aquaria (under 75 gallons) should employ at least 3” of media; larger aquaria will benefit from 6’ (15 cm) or more.

Regardless of where you install your deep sand bed (in the display versus a refugium), you can enjoy the benefits of significant natural nitrate reduction (NNR) and plankton production.

Another significant advantage to deep sand beds is the provision of a dense and natural habitat for numerous micro and macro-organisms. Many fishes and invertebrates cannot be kept successfully or at least optimally without a DSB. Some popular wrasses, jawfishes and gobies… horseshoe crabs, sea cucumbers, stingrays and many other featured creatures will not thrive without thick sandy substrates. Countless invertebrates including some corals will only survive on a sandy seafloor (hard substrates like rock are inhospitable to their feeding strategies and polyp cycles). At various and increasing depths, DSBs provide natural foods for these fishes and reef invertebrates like microcrustaceans (amphipods, copepods, mysid shrimp, etc), bacteria and other nanoplankton. DSBs also provide habitat for desirable plants, algae and animals. Seagrasses like Thalassia and Syringodium (Turtle grass and Manatee grass) for keeping seahorses, pipefish or Cassiopeia (Upside down) jellyfish, for example, require very deep beds at 6” minimum.

The limitations and potential pitfalls of employing a DSB’s are far less mysterious or unpredictable than previously thought. In fact, deep sand bed methodologies now have a history of more than 20 years in use and may fairly claim to be regarded as “tried and true”. Your first decision to make on contemplation of the strategy is purpose. Although you will likely enjoy a combination of benefits with any interpretation of the strategy, some methods are more effective than others in various aspects. Choose from the above described potential benefits and focus on which ones suit you best: aesthetics, nitrate control, or plankton production to begin with. If your purpose for using sand (in contrast to course gravel, shell forms or nothing at all) is only aesthetic, you may wish to forego very deep beds altogether and enjoy a shallow substrate (less then 1”/25 mm) with little regard for sand grain size; there are few benefits or risks in doing so. Nitrate control, instead, is best achieved with sugar fine sand. Zooplankton production (amphipods) to feed fishes may require more coarse sand. And coral propagation (active “fragging” by the aquarist) will often demand a dressing of rubble atop any substrate for a faster settlement of clones and divisions.

After an introduction to the merits of deep sand beds and the “living substrates” you might ask yourself, “what really is live sand?” Live sand is essentially a combination of non-living substrate (usually calcareous in composition but it can be silica-based) with a myriad of tiny beneficial life forms infused throughout it. There are beneficial organisms living on (meiofauna) and between (infauna) the substrate. Creatures found in this medium range from visible zooplankton down to a wide range of microbes dominated by bacteria. Indeed, live sand is much more than microbial colonies battling it out for space and nutrients. All phyla of marine life have representation in sand on the living reef. Some of the most commonly encountered organisms are segmented worms (annelids), roundworms (nematodes), micro-crustaceans (amphipods, copepods, mysids and the like), and bivalves (mollusks), but there are many, many more organisms in tow.

By the activities of live sand organisms, a DSB imports many nutrients, export others, and serves as an extremely efficient living "filter" at large. Space and food are exploited by the colonization and proliferation of crucial microorganisms. Other undesirable elements from the water are simply precipitated and bound into the substrate. Live sand certainly is a complicated and fascinating microscopic world of its own, and quite worthy of a closer look... so get that magnifying glass or microscope out! There is a veritable microscopic zoo to browse.

Plenum Or No?

Since the popularization (and misapplication) of the Jaubert-style plenum for deep sand bed methodologies in the early 1990's, much has been written, debated and revealed about the use of this feature. For those of you new to the issue, rest assured that there is little you truly need to know as a casual aquarist. A plenum is a physical water space underneath a deep bed of sand. It is used to create a hidden, dead space of static water to facilitate the diffusion of nutrients and other vital components of biological processes through the substrate. The premise here is that the biological faculties we seek to harness for natural filtration in live sand can be supported and encouraged by this feature. In the bigger picture, this may indeed true. Some of the challenges of employing this technology in the past were understanding and adapting it from the original recommendations that trickled into popular aquarium literature. It seems that at least some of the early systems incorporating this strategy were very large semi-closed or open systems (fresh flowing seawater) with extraordinary depths of sand that cannot easily be incorporated by home aquarists. Without getting too involved in the "how's" and "why's" of the matter, let us summarize that the plenum methodology has not been demonstrated to be exceedingly useful or particularly harmful. Most aquarists find that there is little difference with or without a plenum for a deep static bed of sand in a healthy home aquarium system. This should bear no reflection on the validity of the methodology, but rather illuminates that the adaptation for home aquarists, especially with smaller aquaria, may have little impact. Admittedly, there are no hard and fast rules here. You may have an interest to experiment with the strategy and are encouraged if so. Just know that having a plenum is not critical to success with deep sand bed methodologies.

One last mention of the improper implementation of plenum and deep sand bed strategies collectively. We should like to dispel the most common corruption of the application for those interested to know or try it. Severe criticism of their use has faulted them for becoming "nutrient sinks": trapping and accumulating detritus to levels that cripple water quality and fuel nuisance algae growths. The reality of the matter may likely be that an incorrect application of the technology caused the rift. As aquarists, we too often have inadequate water flow, which prevents detritus and organic particulates from being properly exported by protein skimming and other filtration dynamics. In turn, excess detritus settles in pockets and migrates deep into the substrate. Furthermore, course sand and gravel is still quite popular and allows particulates to settle and accumulate rather easily. The killing blow to a flawed application with course substrates in weakly circulated aquarium is the unfortunately popular employment of intermediate depths of sand at 1"-3" (25-75mm). In this mid range, the sand is often too deep to be wholly aerobic and yet not deep enough for efficient denitrifying faculties. As such, the two dominant (and desired!) biological populations are restricted if not excluded at large and the sand bed may become a dead zone... a nutrient sink. However, intermediate sand depths can be maintained successfully (often, in fact!), but require due diligence with regular sifting naturally or mechanically (by the aquarist or by creatures in the aquarium), strong water flow in the tank, realistic bio-loads, etc.

Live Sand GALLERY

Indeed, there is no best or ideal grain size for sand in marine aquariums. The selected substrate must serve the needs of the system and its inhabitants. The following is a brief profile of some popular media and their respective merits and limitations:

0.2-1.0 mm Sugar-fine grade... excellent for denitrification and deep sand bed (DSB) strategies seeking natural nitrate reduction (NNR). Ideal at depth for culturing seagrasses like Syringodium and Thalassia. Very supportive of capillary root structures in red mangroves (Rhizophora mangle). Encourages the finest zooplankton (like copepods) to develop in refugia. Ideal substrate for free-living corals like Fungiids (stony mushroom, plate anemone, slipper, tongue and helmet corals), Trachyphyllia (Open brain coral), Goniopora stokesii (Green Flowerpot), and Catalaphyllia jardinei (Elegance). This is the ideal grain size for most detritivores and sand sand-sifting reef animals.

0.2-1.0 mm Sugar-fine grade sand.

* Note: you must be mindful too of the fast rate of dissolution of some sand beds. In most healthy systems, fine aragonite has a "half-life" of 18-24 months. That means that after two years, perhaps, a 3" sand bed will have been reduced to 1.5" and possibly failing in its duties (another unheralded cause for the unfair criticisms of mismanaged DSBs). For this reason, aquarists seeking optimum nitrate control are advised to resist being frugal and apply honestly deep sand beds, and adding more substrate as necessary in time.

1.0mm to 2.0 mm Medium grade sand... similar in form and function to sugar-fine media. Requires slightly greater depth to accomplish denitrification (4-6" minimum recommended). Coarser grain size affords a nice mix of micro-crustacean species to proliferate (e.g. amphipods, copepods and mysids). Calcareous plants flourish here including Halimeda, Udotea and Penicillus. This sand is borderline inappropriate (too large) for sand-sifting detritivores like sea cucumbers and gobies.

1.0mm to 2.0 mm Medium grade sand.

2.0mm to 4.0 mm Coarse grade sand... a challenging grain size to employ for NNR (rather too large in grain to support anoxic faculties if bed is shallow). Requires fairly strong water movement to prevent the buildup of detritus (risk of becoming an unwieldy nutrient sink). Supports larger micro-crustacean populations, like amphipods, very well, but perhaps not the smaller desirable species. Easy to service with a gravel siphon, not as messy or easily disturbed like a sugar-fine grade (.2-1.0 mm). Too coarse for most macroalgae and stony free-living corals. Mismanaged sand beds with this grade have been the single greatest burden on the Deep Sand Bed methodology's reputation.

2.0mm to 4.0 mm Coarse grade sand.

4.0 mm and larger... Very coarse media of any kind over 4mm requires special considerations as a substrate in marine aquariums. Rubble, gravel, and coarse shell demand extra service (weekly siphoning is strongly recommended) and greater attention to flow dynamics and the performance of system hardware will be required (clogging pumps, waning water flow, and skimmer performance- in fear of detritus accumulating to excess). For the lightest burden on your husbandry with this grade, shallow beds work best (1/2" or less, <12mm). Some reef animals will suffer terribly if forcibly kept on coarse media (like some wrasses, eels, sharks and especially rays) as evidenced by developing sores and lesions. Few true plants or macroalgae short of nuisance species will grow well here.

4.0 mm and larger Very coarse media	4.0 mm and larger shell media

Anthony Calfo
The author of The Book of Coral Propagation, Reef Invertebrates, Anthony has been slicing dicing giant leather corals in his Mom's kitchen for years. After his family accidentally ingested a colander of Sinularia "Spaghetti Leather Coral" topped with their favorite tomato and basil marinara, he built an actual greenhouse to hone his propagation and growout techniques. Today Anthony spends most of his time writing and traveling the world, sharing his knowledge and enthusiasm for the hobby, happily branded with his own unique humor and sensibilities.